Proposal: set unlimited max concurrency in PromQL engine

[pracucci]

While working on the query-scheduler documentation (PR) we realised it's quite complex to understand the implications of running a number of query-frontend / query-scheduler replicas higher than -querier.max-concurrent.

Given we assume a Grafana Mimir cluster always run with the query-frontend and given the max concurrency per querier is enforced by the number of running workers (because each worker can only execute 1 query at a time), we could simply configure the PromQL max concurrency to "unlimited".